DSL filters are a source of some amusement to me.
I had a client that was trying to trouble shoot DSL. It was working, but running as slow as dial up. They had spent hours on the phone with their provider and had no success. When I got there I checked a few settings in Windows then back tracked the cabling from PC to wall looking for kinks. Everything looked great until I got to the phone jack. They had piggybacked two filters!
Removed one and everything was back to normal. Father had set it up originally and the son (no comment) added the second one when they were having problems. Of course he never said a word to dad about it.